There is an increasing demand for rectangular chip resistors with highly accurate resistance to eliminate adjustment for circuits, as the size of electronic equipment continues to shrink in recent years. In particular, since the allowance required for the resistance of rectangular chip resistors is .+-.0.1% to .+-.0.5%, the demand for rectangular chip resistors made of thin metal film resistance material (hereafter referred to as "thin film rectangular chip resistors"), in which precise resistance can be easily achieved, is overtaking demand for conventional rectangular chip resistors, which are constituted of thick film resistance (hereafter referred to as "thick film rectangular chip resistors") made in grazed material.
On the other hand, as the use environment of electronic equipment diversifies, the required specification levels for rectangular chip resistors, which are electronic components, is also becoming higher. As the market for thin film rectangular chip resistors expands, reliability equivalent to that of thick film rectangular chip resistors, which have stable moisture resistance characteristics, is required.
A resistor and its manufacturing method of the prior art are explained below with reference to a drawing.
As shown in FIG. 4, a resistance layer 2 made of a thin metal film of Ni or Cr systems is disposed on the top face of a substrate 1 made typically of 96% aluminum. A pair of top electrode layers 3 made of a thin metal film such as Cu etc. are disposed on the left and right ends of the top face of the substrate 1 so as to overlap the resistance layer 2. A pair of bottom electrode layers 4 made of a thin metal film such as Cu etc. are disposed on both ends of the bottom face of the substrate 1, at positions corresponding to the top electrode layers 3. A protective layer 5 typically made of polyimide resin is provided on the top face of the resistance layer 2 to cover at least an exposed area of the resistance layer 2. In addition, a side electrode layer 6 made of a thin metal film such as Ni etc. is disposed on side faces of the substrate 1 so as to connect the top electrode layer 3 and the bottom electrode layer 4. Lastly, a Ni plating layer 7 is provided to cover the top electrode layer 3, bottom electrode layer 4, and side electrode layer 6. A solder plating layer 8 is provided to cover the Ni layer 7 to form a complete resistor.
A method for manufacturing the resistor as configured above is explained next with reference to a drawing.
FIG. 5 is a process chart showing a method for manufacturing the resistor of the prior art. A substrate 11 is a heat-resisting substrate made typically of 96% aluminum (Process A). A thin film resistance layer, typically of NiCr etc. is provided on the entire face of the substrate 11 by sputtering (Process B). A resistance pattern 12 is formed by photo-etching this thin film resistance layer (Process C).
Next. a thin film top electrode layer such as Ni etc. is sputtered on the entire face of the substrate 11 where the resistance pattern 12 is formed (Process D), and a top electrode pattern 13 is formed by photo-etching this thin-film top electrode layer (Process E). Then, heat treatment at 350.degree. C. to 400.degree. C. is applied in a nitrogen gas ambient to stabilize the films of the resistance pattern 12 and the top electrode pattern 13 (Process F).
Next, laser trimming is applied to adjust the resistance of the resistance pattern 12 to a specified value (Process G). A protective layer 15 made of thermosetting resin such as polyimide resin is provided to protect the resistance 14 after the resistance is adjusted (Process H).
Next, a groove 16 for dividing the substrate 11 is made by scribing with carbon oxide gas laser (Process I), and the substrate 11 is primarily divided to substrate strips 17 (Process J). A side electrode layer 18 is formed on a side face of these substrate strips 17 by means such as sputtering (Process K).
After secondary division of the substrate strips 17 into substrate pieces 19 (Process L), an electrode plating layer 20 is finally formed to secure reliability of soldering (Process M), resulting in manufacture of the resistor of the prior art.
The resistor and its manufacturing method of the prior art, however, use thermosetting resin such as polyimide resin for the protective layer of thin film rectangular chip resistors. This has far greater water vapor permeability, due to its material characteristics, comparing with inorganic materials such as the borosilicate lead glass used as the protective layer for thick film rectangular chip resistors. Accordingly, water molecules are likely to penetrate the resistance layer through the protective layer if the resistor is exposed to a high ambient humidity. This will cause changes in resistance value due to oxidization of the resistance layer. Furthermore, electro-corrosion may cause disconnection if ions with high corrosivity such as Na.sup.+, K.sup.+, and Cl.sup.- are present.